关系库执行计划中三种最常用连接方式的伪码实现

1、NL

for each row R1 in the outer row source
for each row R2 in the inner row soruce
if R1 joins with R2
return (R1, R2)

2、merge

get first row R1 from input 1
get first row R2 from input 2
while not at the end of either input
begin
if R1 joins with R2
begin
output (R1, R2)
get next row R2 from input 2
end
else if R1 < R2
get next row R1 from input 1
else
get next row R2 from input 2
end

3、

for each row R1 in the build row source
begin
calculate hash value on R1 join key(s)
insert R1 into the appropriate hash bucket
end
for each row R2 in the probe row source
begin
calculate hash value on R2 join key(s)
for each row R1 in the corresponding hash bucket
if R1 joins with R2
output (R1, R2)
end